segmentation fault

Hello,

I am using reverb 0.4.0 in tf-agents 0.9.0 through the `ReverbReplayBuffer` and `ReverbAddTrajectoryObserver`. Ray actors push experience to the the reverb server. Currently though, it is configured to have just 1 actor, and experience pushing is completed in a blocking manner before agent training in the main loop.

I am seeing segmentation faults happening at random times, always within the main process that samples from reverb to train the agent.

I was wondering if you have some hints about why this might be happening, or where I can start debugging?

```bash
*** SIGSEGV received at time=1629367150 on cpu 4 ***
PC: @ 0x7f328092219f (unknown) deepmind::reverb::(anonymous namespace)::LocalSamplerWorker::FetchSamples()
@ 0x7f32d0810980 2320 (unknown)
@ 0x7f328091a14f 144 deepmind::reverb::Sampler::RunWorker()
@ 0x7f32cd7cd039 (unknown) execute_native_thread_routine
@ 0x7f2be000ec70 (unknown) (unknown)
@ 0x7f3280965ca0 (unknown) (unknown)
@ 0x75058b4808ec8348 (unknown) (unknown)
Segmentation fault (core dumped)
```